The present application relates to a throttle-cycle cooler operating with mixed refrigerant (MR) that provides functionality of instruments in the field of thermal analysis. Specifically, a differential scanning calorimeter (DSC) is a basic instrument used in thermal analysis.
Thermal analysis is widely used in various applications including the pharmaceutical and food industries, material science, electronics, etc. The analysis is based on a comparison of the calorimetric properties of a sample to be investigated, analysis sample (AS), and a reference sample (RS) with calibrated properties.
The samples are placed in containers located on a plate, with plate temperature regulated in a range of TMAX to TMIN. Any temperature in this range is supported precisely by the temperature control system, which regulates power of a heater attached to a hot plate as shown in FIG. 1. A cooling system incorporated in the DSC provides a cold plate temperature. The cold plate is connected to the hot plate with a thermal bridge that is a thermal resistor. The balance between input heater power and cooling capacity of the cooling system provides accuracy of measurements and rapid transition time from one temperature point to another in a typical temperature range of TMAX=700 C. (973K.) to TMIN=xe2x88x92150 C. (123K.), and xe2x88x92195 C. (78K.) in some application. Existing cooling systems provide TMIN that depends on the field of application of the thermal analysis, such as:
TMIN=xe2x88x92170 C. for glass transaction, polymorphism, purity, dynamic mechanical analysis;
TMIN=xe2x88x92150 C. for thermo-mechanical analysis, expansion coefficient, penetration;
TMIN=xe2x88x9250 C. (223K.) for laser flash methods: thermal diffusivity of ceramics, alloys, fibers;
TMIN greater than xe2x88x9250 C. for thermal gravimetry.
A cooling system (CS) in a current design might be one of different types depending on the temperature range that is selected for analysis. For example, air cooling is useful through a range +50 C. (323K.) to +100 C. (373K.). Closed-cycle coolers that use conventional refrigerants may be used when providing temperatures down to xe2x88x9250 C., and a system based on liquid nitrogen can provide cold plate temperatures down to xe2x88x92195xc2x0 C.
A major disadvantage of existing cooling systems employed in a DSC below xe2x88x9250 C. is that they depend on a liquid nitrogen supply and boil-off system. Such a system increases the operating cost of the DSC. The electronic temperature control system is complicated because of a high temperature difference between the hot plate and cold plate. In addition, refilling required for liquid nitrogen use does not allow continuous long-term operation for automatic analysis of samples.
An object of this invention is to provide a closed-cycle cooler to support functionality of the thermal analysis instruments and specifically a DSC at minimal temperature below room temperature down to xe2x88x92183 C. (90K.)
Another object of this invention is to develop a cooler that easily allows a customized interface with thermal analysis instruments including DSCs of different configurations to operate in a predetermined temperature range.
Generally speaking, in accordance with the invention, a cooling system for thermal analysis equipment and the like provides rapid cool down and steady state operation of a differential scanning calorimeter (DSC) at any predetermined temperature between a minimal temperature and room temperature using a throttle-cycle cooler based on a single stage compressor. In the cooling system, the cooler operates with a mixed refrigerant that may include some liquid fraction at the inlet to a cryostat that houses the key cold elements for the cooling system. A temperature actuated throttle valve in the cooling system increases refrigerant mass flow rate when the differential scanning calorimeter increases the heat load, generally provided by a heater. At the same time, the valve design provides a high mass flow during cool down, and automatic flow rate reduction at an intermediate temperature as the overall system approaches an operating condition after cool down.
Still other objects and advantages of the invention will be apparent in the specification.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts, which will exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.